From US patent publication US 2004/0107561 a plasma generator is known wherein the plasma source comprises a RF antenna generating a plasma, and wherein an electromagnetic shield is applied.
In general, plasma particles comprise electrons, ions and radicals. Electrons and ions are charged particles, so that the interaction with a surface to be cleaned depends on a charge distribution present at the mentioned surface. The electromagnetic shield counteracts a flow of charged plasma particles flowing towards the object, so that mainly merely radicals reach the surface to be cleaned. As radicals are electrically neutral, an electrical interaction depending on any charge distribution on the surface to be cleaned is avoided and a more reproducible and reliable cleaning operation is performed. Moreover, sputtering effects are avoided.
However, in a process of cleaning irregular 3D-structures, e.g. for medical sterilization purposes, homogeneity performances are weak. As an example, an object section near the plasma source might be etched too rigorously, while another object section located more offset to the plasma source might undergo a cleaning process that does not satisfy predetermined cleaning requirements. Obviously, such an inhomogeneity in cleaning performance might be unacceptable for certain applications.